The invention relates to cyclic organometallic compounds which contain aluminum, gallium or indium as metals and also to the use of said compounds for producing thin films or epitaxial layers by gas-phase deposition.
The deposition of such layers composed either of pure elements of group III or of combinations with other elements, such as, for example, gallium arsenide, indium phosphide or gallium phosphide, may be used to produce electronic and optoelectronic switching elements, compound semiconductors and lasers. Such layers are deposited from the gas phase.
The properties of these films depend on the deposition conditions and the chemical composition of the film deposited.
All the known methods, such as the Metal-Organic Chemical Vapor Deposition (MOCVD) Method, the Photo-Metal-Organic Vapor Phase (Photo-MOVP) Method in which the substances are decomposed by UV irradiation, the Laser Chemical Vapor Deposition (Laser CVD) Method or the Metal-Organic Magnetron Sputtering (MOMS) Method are suitable for the deposition from the gas phase. The advantages over other methods are a controllable layer growth, a precise doping control and also easy handling and convenience of production owing to the normal-pressure or low-pressure conditions.
In the MOCVD Method, use is made of organometallic compounds which decompose at a temperature below 1100.degree. C. to deposit the metal. Typical apparatuses which are at present used for MOCVD comprise a "bubbler" with an inlet for the organometallic component, a reaction chamber which contains the substrate to be coated, and also a source for a carrier ga which should be inert towards the organometallic component. The "bubbler" is kept at a constant, relatively low temperature which is preferably above the melting point of the organometallic compound but far below the decomposition temperature. The reaction or decomposition chamber is preferably at a very much higher temperature which is below 1100.degree. C., at which temperature the organometallic compound decomposes completely and the metal is deposited. The carrier gas converts the organometallic compound to the vapor state and the latter is then channelled along with the carrier gas into the decomposition chamber. The mass flow of the vapor can be controlled satisfactorily, and consequently, controlled growth of the thin layers is also possible.
Hitherto, metal alkyls such as, for example, trimethylgallium, trimethylaluminum or trimethylindium have mainly been used for the gas-phase deposition. These compounds are, however, extremely sensitive to air, spontaneously ignitable and in some cases are decomposable even at room temperature. Elaborate safety measures are therefore necessary for producing, transporting, storing and using these compounds. A few, somewhat more stable adducts of the metal alkyls with Lewis bases such as, for example, trimethylamine and triphenylphosphine are also known (described, for example, in GB Patent Specification 2,123,422, EP-A-108,469 or EP-A-176,537), but these have only limited suitability for gas-phase deposition owing to the low vapor pressure.
Furthermore, similar compounds are described in German Offenlegungsschrift 3,631,469 but these do not have a cyclic structure with the metal in the cycle.